Most of the women I see in my gym spend hours on cardio and very little lifting weights, trying to lose weight and get fit. While cardio is great, I often wonder why they don't spend more time lifting. Saw this article today and it gives a good reason why they should lift.

U.S. scientists have used muscle-bound mice to show how weight training may be just as effective as aerobic exercise in melting away fat.

While the mice did not actually pump iron, their genetically modified muscles were made to resemble those found in bodybuilders.

And while that type of "brawny" muscle was thought to have little capacity to burn fat, the study suggested its presence in the body could melt away pounds as readily as aerobic activities.

"It was really very surprising ... to see the weight loss that was associated with those muscles," says Boston University researcher Kenneth Walsh, the lead study author.

"What it tells people ... is that they should consider exercise programs that involve both aerobics as well as weight training," says Walsh, head of molecular cardiology at the school's Whitaker Cardiovascular Institute.

The study was published today in the journal Cell Metabolism.

Weight training typically builds what is known as type II or "fast-twitch" muscle tissue, Walsh says, which contains low concentrations of energy-burning mitochondria.

Type I, or slow-twitch muscles, are built up through endurance exercises like running and are rich in the intercellular energy plants.

It was believed this advantage made type I muscles far more efficient at burning fat than their bulkier counterparts.

"What most people traditionally thought was the more type I muscle you had, the more mitochondria you had, the bigger your furnace, the more you burn the fat," says Dr. Mark Tarnopolsky, professor at the department of medicine at Hamilton's McMaster University.

But Tarnopolsky, who has studied the effects of weightlifting on the elderly and was commenting on the study, said it pointed to something unrelated to muscle mitochondria as a fat-burning mechanism.

While bulky, weight-training muscles are not burning fat, it may well be that they are in communication with other organs to create energy for them, Walsh says.

In particular, he says, the type II muscles may be directing the liver to turn on its fat-metabolizing engine, one of the body's most powerful weight-control tools.

"And what that's telling us is that there are signalling molecules produced by this brawny muscle that say to other tissues like the liver, `I'm growing, I need more energy.'"

The mice, obese to begin with, were expected to grow into sumo wrestler-like rodents. Instead, they experienced weight loss of some 40 per cent within three weeks of developing bodybuilder-type muscles, Walsh says.

Researchers manipulated the mice into bulking up by switching on a gene – known as Akt1 – that promotes type II muscle growth, but not its aerobic counterpart.

Although they looked only slightly more muscular than your average rodent, Walsh says tests showed most of the muscle they did have was of the fast-twitch variety.

Both scientists say the weight-training muscle may also help control diseases like Type 2 diabetes and heart ailments.

"Weight training has been sort of underappreciated in terms of its importance," Walsh says. "This study shows those muscles have whole body systemic effects."

Most of the women I see in my gym spend hours on cardio and very little lifting weights, trying to lose weight and get fit. While cardio is great, I often wonder why they don't spend more time lifting.

I feel there are 3 reasons why most women don't lift weights. The first one being they are not educated about weightlifting; fearing that it could make them bulk up and look manly. It doesn't help all the other uneducated wimps talking them out of it as well. At the gym, I get more compliments on my physique from womenand very few from men, most of the remarks I get from men are "damn you're buff" (surprised look on face) and "you look like you can kick my ass" (while squeezing my biceps with their thumb & forefinger). I can't imagine what other unencouraging things guys might say to other women (i'm sure they're not intentionally trying to discourage us)

The second being they don't know where to start; with all that fancy equipment, I can see how a newbie can be confused and intimidated. If not the grunting, smelllllly meatheads, then the toned princesses who prance around in their brazilian gear in cranial armor and war paint.

Third being that lifting weights actually requires exertion and it's easier to use a cardio machine. How many of us see people on treadmills walking at a 2.5mph and never break a sweat? Just the other day, I was on the stairmill next to a chick reading her romance novel on level 2 (while I was pushing 2-minute, level 4 and 10 intervals) ! I didn't feel very feminine next to her, but I guarantee she has way more cellulite than me.. hehehe